Cellulosic and lignocellulosic feedstocks and wastes, such as agricultural residues, wood, forestry wastes, sludge from paper manufacture, and municipal and industrial solid wastes, provide a potentially large renewable feedstock for the production of valuable products such as fuels and other chemicals. Cellulosic and lignocellulosic feedstocks and wastes, composed of carbohydrate polymers comprising cellulose, hemicellulose, and lignin are generally treated by a variety of chemical, mechanical and enzymatic means to release primarily hexose and pentose sugars, which can then be fermented to useful products.
Pretreatment methods are used to make the carbohydrate polymers of cellulosic and lignocellulosic materials more readily available to saccharification enzymes. Standard pretreatment methods have historically utilized primarily strong acids at high temperatures; however due to high energy costs, high equipment costs, high pretreatment catalyst recovery costs and incompatibility with saccharification enzymes, alternative methods are being developed, such as enzymatic pretreatment, or the use of acid or base at milder temperatures where decreased hydrolysis of biomass carbohydrate polymers occurs during pretreatment, requiring improved enzyme systems to saccharify both cellulose and hemicellulose.
Teixeira, L., et al. (Appl. Biochem. and Biotech. (1999) 77-79:19-34) disclosed a series of biomass pretreatments using stoichiometric amounts of sodium hydroxide and ammonium hydroxide, with very low biomass concentration. The ratio of solution to biomass is 14:1.
Elshafei, A. et al. (Bioresource Tech. (1991) 35:73-80) examined the pretreatment of corn stover utilizing NaOH.
Kim, T. and Y. Lee (Bioresource Technology (2005) 96:2007-2013) report the use of high amounts of aqueous ammonia for the pretreatment of corn stover.
Int'l. Pat. App. Pub. No. WO2004/081185 discusses methods for hydrolyzing lignocellulose, comprising contacting the lignocellulose with a chemical; the chemical may be a base, such as sodium carbonate or potassium hydroxide, at a pH of about 9 to about 14, under moderate conditions of temperature, pressure and pH.
U.S. Pat. Nos. 5,916,780 and 6,090,595, describe a pretreatment process wherein a specified ratio of arabinoxylan to total nonstarch polysaccharides (AX/NSP) is assessed and used to select the feedstock.
U.S. Pat. No. 5,196,069 discloses a process for converting cellulosic waste into soluble saccharide by irradiating an aqueous cellulose feed mixture with microwave radiation in the presence of acetic acid at an elevated pressure, the efficiency obtained from an enzymatic hydrolysis is greatly enhanced.
Most pretreatments such as the ones described above either result in a pretreated biomass depleted of lignin and hemicellulose or the partial depletion of hemicellulose with retention of most of the lignin. Therefore a method is needed to selectively remove only lignin without significant loss of either hemicellulose or cellulose from the biomass, as these constitute the source of sugars for fermentation.
Most approaches to converting polysaccharides to a source of fermentable sugars have relied on the use of either acid catalyzed hydrolysis or enzymatic saccharification for the hydrolysis of xylans and glucans to monosaccharides. The acid-only based approach suffers from both the low yield often seen in acid-catalyzed hydrolysis and also the generation of byproducts which can be detrimental to down stream processing steps, such as fermentation. This arises from the vast difference in the kinetics of hydrolysis of xylans and glucans, which are more difficult and easier to hydrolyze, respectively. The difference in stability of the sugars when heated under acidic conditions is also a drawback to the acid-only approach. Furthermore, the presence of acid or its salt, especially of organic acids, can result in lower performance of fermentation enzymes, necessitating the removal of the organic acid or its salt prior to the fermentation of the hydrolyzate. The enzyme-based approach suffers from the high cost associated with enzymes and the recalcitrance of the biomass to undergo quantitative saccharification. A method of converting polysaccharides to monosaccharides which overcomes these difficulties is needed.